The present invention relates to a method of controlling the idle speed of an internal combustion engine.
A known method of controlling the idle speed consists of controlling by closed-loop the opening degree of a throttle valve or the opening degree of a flow-control valve in a by-pass intake passage which is provided in parallel with an intake passage that accommodates the throttle valve, by using a valve-control motor, such as a step motor or a servo motor, depending upon the running speed when the engine is in the idling condition. According to this known idle-speed control method, the engine speed and a desired idle speed are compared with each other and the valve-control motor is adjusted to control the idle air flow depending upon the compared result so that the engine speed is converged by closed-loop to the desired idle speed.
The above-mentioned closed-loop idle speed control is executed only when the operating condition of the engine enters a predetermined idling condition. The predetermined idling condition is when the throttle valve is at the idle position and the running speed of the vehicle mounting the engine is nearly zero.
However, closed-loop idle speed control is sometimes carried out by mistake even if the operating condition is not the predetermined idling condition. In such a case, if the throttle valve suddenly returns to the fully closed position, the rotational speed of the engine abruptly decreases. In the worst case, the engine stalls.
For instance, during deceleration, if the vehicle speed sensor malfunctions and outputs an error signal which indicates the vehicle speed is zero, the closed-loop control system deems that the engine is in the predetermined idling condition and thus executes the idle speed control. In this case, since the engine speed is high, the idle opening degree of the throttle valve or the opening degree of the bypass flow-control valve is reduced. Therefore, if the transmission is shifted to the neutral position, the engine speed abruptly decreases, causing the engine to stall.
Furthermore, there occurs a problem if the throttle position switch is a type which produces an idle position signal even when the throttle valve opens by a certain extent and also if the vehicle speed is zero. In this case, the closed-loop idle speed control is executed to decrease the idle opening degree of the throttle valve or the opening degree of the bypass flow-control valve when the throttle valve gradually opens from its fully closed position. During such a state, if the throttle valve suddenly closes, since there is not sufficient idle air flow, the engine speed abruptly decreases, causing the engine to sometimes stall.